Deleterious Effects Of Low Velocity Flow In Piping And Pipelines
This topic however deals with low velocities. The adjective I have used is deleterious for low velocities. The reason being that, while high velocities can have an immediate impact in terms of noise or vibrations in piping systems, the undesirable effect of low velocities in piping is more subtle and long-term. The adjective deleterious signifies this subtle and long-term effect of low velocities.
Let
us come to those deleterious effects of low velocities in piping and pipelines.
I will categorize the flow in pipes and pipelines based on the type of fluid
and the fluid phase.
Flow of Slurries (liquid-solid homogeneous
phase)
At very low flowing velocities, phase separation
will occur of the solid particles, with the higher density solid particles
tending to settle down at the pipe bottom of a horizontal pipe run. The
settling will be even more at bends (direction change), or where there is a
reduction in the pipe diameter. The deleterious effect is that over a period of
time a layer of solid builds in the pipe and pipe fittings, reducing the pipe
diameter leading to a) excessive pressure drop b ) partial or full disruption
of flow c) pump operating point moving towards shut-off leading to reduced pump
efficiency and accelerated mechanical wear and tear of the pump and pump
sealing system.
Flow of Liquids (comprising of entrained
heavier liquid with a lighter liquid)
A typical example would be a hydrocarbon liquid
with entrained water, where the hydrocarbon liquid is the continuous phase
while the water is the discontinuous phase. At low flowing velocities the
entrained discontinuous phase water will drop out from the continuous phase
hydrocarbon liquid to the bottom of the pipe. Water accumulation will occur in
low points of the piping over a long term. If the hydrocarbon liquid has even
trace amounts of dissolved carbon dioxide or hydrogen sulfide, pipe / pipeline
corrosion can occur. The mechanism of corrosion in simplistic terms is that
carbon dioxide and / or hydrogen sulfide will react with the accumulated water
in the pipeline leading to acid corrosion by formation of Carbonic Acid (H2CO3)
and / or Sulfuric Acid (H2SO4). Such corrosion can lead
to failure of carbon steel piping / pipeline over a long term.
Single Phase Gas Flow (with entrained liquids)
A typical example would be natural gas
containing entrained liquid droplets of water and heavier hydrocarbons. At low
flowing velocities the flow would be stratified in the horizontal pipe, where
the gas travels at the top of the pipe and the liquid travels at the bottom of
the pipe and liquid accumulation occurs at low points and direction changes in
pipe over a long period of time. The same problem of corrosion can occur as discussed
for flow of liquids. Additionally, the gas transport could see high pressure
drops and reduction in flow, putting excessive loads on gas compressors. Liquid
accumulation over a long term in pipelines due to low velocities can also lead
to intermittent slug flow in pipelines. The high momentum of liquid slugs can
lead to structural damage of piping / pipelines and their supports.
Often natural gas pipelines have been found to
have black powdery material (solids) in small amounts. The black powder could
be because of corrosion products, trace amounts of solids carried over from gas
treatment plants, mill scale etc. Low flowing velocities in gas transmission
pipelines can lead to accumulation and deposition on pipeline walls of the
black powder over a long-term and lead to excessive pressure drop and reduced
flow. Flow velocities need to be kept above a threshold velocity also known as
minimum entrainment velocity to prevent black powder deposits.
3-phase flow (Gas-Liquid-Liquid with Oil as
continuous phase)
A typical example would be crude oil from
reservoirs with associated gas (dissolved or free) and free water. Low flowing
velocities will cause similar problems as discussed above related to corrosion.
Additionally, if the crude oil is heavy crude oil containing asphaltenes, then
reduced flow velocities (reduced flow) for a given pipeline will drastically
increase the asphaltene deposition rate on the pipe walls. The long-term effect
could be partial or total flow stoppage requiring costly cleaning operations
for full restoration of pipeline operations.
Quantification of Minimum Velocities
I had prepared a standard “Specification for
Process Design Basis” for a middle-east oil & gas operating company wherein
I had mentioned also about minimum velocities in pipelines. For unlined carbon
steel pipelines transporting liquid hydrocarbons (light crude oil or
condensate) containing free or entrained water even in a small quantity (e.g.
1% water cut), the velocities should not be allowed to fall below 1.5 m/s to
prevent water dropout. For gas pipelines I had mentioned that the normal range
of flow velocities should be 5 to 10 m/s. For bone dry gas, velocities up to 20
m/s may be allowed, subject to design considerations for noise and vibration
prevention during all operational scenarios.
As mentioned earlier, black powder in natural
gas pipelines will not be entrained at low gas velocities. This may lead to
accumulation at some portion of the gas pipeline over a long duration. The
entrainment velocity for these very fine particles is a function of their
micron size. Generally, for a particle size of 1 micron to remain entrained,
the gas velocity should be in the range of 2.5 to 4.5 m/s, depending on the
pipe size.
Measures to prevent low velocities
In intermittent or batch transfer operations,
when demand is low at the receiving station, continue pumping at the same high
rate as required for peak demand but for a shorter duration. There is no need
to reduce the flow rate for low demand in batch transfer operations. Standard
Operating Procedures (SOPs) should address such turndown or low demand scenario
without reducing the flow and thus the velocity.
Measures to mitigate low velocities
In continuous operations and prolonged turndown
scenarios, low flow velocities are unavoidable. To prevent the aforementioned
problems associated with low flow velocities, some measures that could be
implemented are described below:
1. Injection of anti-corrosion and anti-scale
additives in the piping / pipeline system.
2. Addition of emulsifying agents in liquid
hydrocarbon-water systems to prevent phase separation occurring at low
velocities.
Note: The addition of additives to
prevent corrosion, deposits and phase separation should be carefully evaluated
from the viewpoint of chemical compatibility with the process fluid and any
adverse effects in downstream applications.
To conclude, low flowing velocities in pipes and
pipelines can create problems such as corrosion, scaling and deposit formation
and design and operational measures should be considered to prevent operations
at low velocities.